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Keeping volunteer canola out of soybeans

Inter-row tillage looks like a promising management tool

The average of six per cent canola seed loss during harvest sounds bad enough, but even worse when converted into the number of seeds left to germinate as volunteers the following year.

“We’re losing an average of 4,000 to 5,000 seeds per square metre so it doesn’t take a lot of persistence for volunteer canola to be an issue,” Robert Gulden of the department of plant science at the University of Manitoba told the Manitoba Agronomists Conference in December.

If soybean is the following crop, reducing the seed bank is critical because it’s much less competitive than canola.

In studies covering 300 Prairie canola fields over three years, Gulden and other researchers have found that anything which increases yield, such as fertility or spraying for sclerotinia, reduced the proportional harvest losses.

Gulden said higher seeding rates also help make soybeans more competitive against volunteer canola and protect yield.

“In one study we are looking at narrow row spacing of 7.5 inches and a seeding rate of 105 kilograms per hectare, targeting 180,000 plants per acre, which is 1.5 times higher than the recommended seeding rate for soybeans,” Gulden said.

“We get higher yields with higher seeding rates, so plant numbers matter and the soybeans were able to compete better with the volunteer canola and pushed down our seed return numbers.”

Management begins at seeding

Managing volunteers begins with seeding the canola. Gulden’s research shows that harvesting earlier lowers seed losses during harvest, and ensuring an earlier harvest has a lot to do with plant density.

“When we seeded canola at 80 plants per square metre and 20 plants per square metre the heavier seeding rate matured more evenly and led to earlier maturation than the lower seeding rate,” Gulden said.

Slowing down the combine can also help reduce losses. “The faster we drive the combine the greater the harvest losses,” he said.

Early-fall tillage

Because a lot of volunteer canola is also herbicide resistant, it can be difficult to control. Gulden has found that an early-fall tillage pass is the best way to reduce the canola seed bank.

“With zero till we saw very little fall seedling recruitment, but with an early fall tillage pass, shortly after harvest, we can encourage a number of volunteer canola seedlings to emerge and in most winters will be killed, and won’t be in the seed bank the following spring,” Gulden said. “If we wait to do a late-fall tillage the result looks similar to zero till.”

He said in-season inter-row tillage between wider row spacings may help deal with volunteers. “We compared inter-row mulches of wheat and fall rye between soybeans on 30-inch row spacings with inter-row tillage. The mulches did not appear to affect yield or the seed return very much, but the inter-row tillage looks promising from an integrated weed management perspective because it gave us higher soybean yield and a lower volunteer canola seed return,” Gulden said.

“Keeping in mind what’s happening with glyphosate resistance in the U.S., I would encourage anybody who is growing wide-row soybean to use inter-row tillage as a weed management tool, just to help avoid some of those issues that are coming our way.”

Heavy losses

Because volunteer canola is so competitive, the action threshold in soybeans is low — between 1.2 to 2.8 plants per square metre, after which producers can expect to see a five per cent yield loss.

Gulden and other researchers across Western Canada are conducting studies to see how effective herbicides are against volunteer canola.

“There are a lot of herbicide options available, and some work better than others but the best choice depends very much on the situation,” Gulden said.

He has found that timing of application to coincide with the critical weed-free period is essential for herbicides to be most effective.

“What’s important to preserve yield is how quickly the herbicide works because hitting the critical weed-free period is crucial to get good control,” Gulden said.

But the problem is no one knows yet what the critical weed-free period for soybeans is in Manitoba.

“Group 2s take two to three weeks to work, during which time the volunteer canola is still growing and still competing with the slower-growing soybean. Products that work very quickly and are applied when the critical period starts may do a much better job of preserving yield,” Gulden said.

“As little as soybean grows in our conditions during the month of June, our data shows it is still very critical what happens in that time frame in terms of forming yield.”

In Ontario production guides suggest the critical weed-free period for soybeans is the vegetative V1 to V3 growth stages, but Gulden suspects in Western Canada it may be much later.

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